4.2 The Black Hole Information Puzzle Review

summary

You may have noticed that two completely opposite assertions have been stated.

Black holes are the simplest objects in the universe—less characteristic than even empty space. And yet at the same time, according to Stephen Hawking, they contain a maximal amount of information. So a black hole at once embodies the simplest and most complex thing in our universe.
The fact that we arrive at these two different pictures is a wonderful example of a contradiction. How can we reconcile this?
String theory is a candidate for reconciling theories with both quantum mechanics and gravity, and if it truly does achieve this goal, then it must somehow give an answer to where the information is stored in a black hole—it’s obligated to do that.

Enter string theory.

String theory was first discovered in the 1970s with the idea that all particles, on closer inspection, are made of strings. These tiny loops of string, by rotating and vibrating in different ways, can “disguise” themselves as all of the different elementary particles.
Since the 1970s, the theory has progressed into a large and highly cohesive framework, incorporating nearly all approaches to beyond-the-Standard-Model physics. We have learned that strings are just one of many equivalent descriptions, but the name “string theory” has stuck.
No one will be confident that string theory is the theory of nature until we have direct experimental confirmation—and since strings are so small, it is unlikely we will get such confirmation in the foreseeable future.
Nevertheless, we can use string theory as an example of a consistent theory of quantum gravity and try to understand at least one way that this black hole information puzzle can be solved.

String theory provided one of the first reconciliations of quantum gravity.

In 1996, string theory was used to give a complete microscopic quantum description of a class of especially simple supersymmetric black holes, in a seminal paper by Andrew Strominger and Cumrun Vafa.
They found that the horizon of a black hole is kind of like a hologram because it has a fully equivalent, lower-dimensional description.
A hologram (or holographic plate) is a two-dimensional plate which can project a three-dimensional image.
Strominger and Vafa concluded that the information stored in the lower-dimensional, holographic description is exactly equal to the Bekenstein-Hawking entropy.
This was the first time since the information puzzle arose that this entropy formula had ever been reproduced by actually counting quantum microstates.
The maximally simple (higher-dimensional) and maximally complex (lower-dimensional) descriptions of black holes were found to be complimentary and perfectly compatible.